Hydrothermal Stability of Modified Silica Membranes for Gas Separation

The hydrothermal stability of molecular sieve silica (MSS) membranes is an area of increasing research interest, in particular for applications such as membrane reactors and membrane gas separation of process streams containing hot steam. In these applications, gas streams are likely to contain water vapor, which reacts with the hydrophilic sites in the silica thin films resulting in chemical and microstructural instability [1]. Various research groups have produced high quality membranes using a single-step catalysed hydrolysis [2,3,4] or a two-step catalysed hydrolysis sol-gel process [5,6]. Sol-gel derived films, which contain a large amount of silanol groups have pore sizes with molecular dimensions in the region of 3-4Å. Hence these materials are ideal precursors to synthesize membranes for gas separation. However, silanol groups are hydrophilic and easily react with water molecules, resulting in further changes in the matrix of silica-derived materials. Fotou et al. [7] reported that complete densification of pure silica membranes occurs at 800C in a 50% mol steam atmosphere. Novel synthesis methods designed to increase the hydrophobicity of MSS membranes include thermal treatment or surface modification using template agents. However, such methods may alter the micropore structure of the materials and hinder their molecular separation properties. Templates can be classified as organic covalent ligands (such as methyl groups [8]) or non-covalently bonded (such as surfactants [6]). De Vos et al. [8] recently embedded methyltriethoxysilane (MTES) into a sol-gel process prepared with tetraethylorthosilicate (TEOS), water, ethanol and acid. This process introduces methyl groups to the silica matrix as template agents to enhance hydrophobicity. These membranes showed good permeances while selectivities decreased due to a broader pore size distribution compared with membranes prepared without MTES. Various research groups have used non-covalently bonded organic templates, such as C6 and C16 surfactants [6] and alkyltriethoxysilanes [9], to tailor the pore size of intermediate or top layers of membranes. Surfactant templated silica samples have a high porosity and narrow pore size distribution and therefore minimize flow resistance, surface roughness, and inherent support defects. A surfactant is selected by its length which influences the final pore size of the silica, the interaction with the solvent and the position of the silica network it should interact with in order to form pores and to increase the hydrophobicity. Giessler et al [1] recently reported hydrophobicity results of three types of xerogels: (i) Hydrothermal Stability of Modified Silica Membranes for Gas Separation